Upgrading a hypervisor can involve shutting down the virtual-machines hosted by the hypervisor. Depending on the mission(s) to which the virtual machines have been dedicated, the shutdown may be costly or otherwise unacceptable. To avoid the shutdown, the virtual machines can be migrated to a standby machine, e.g., using a product such as vMotion, available from VMware, Inc. For example, when upgrading ESX, available from VMware, Inc., the host is put in a maintenance mode that will migrate all the virtual machines from the host machine to a standby machine. While the virtual machines execute on the standby machine, the original host machine can be provided with an upgraded hypervisor. The virtual machines can be migrated back, completing the upgrade. Of course, if the standby machine has an instance of the upgraded hypervisor, the return migration may be omitted.
Relying on migration to a standby machine to avoid shutting down virtual machines can be problematic. First of all, the required standby machine may not be available. Also, depending on the number of virtual machines and/or their average size, each migration may consume considerable network bandwidth for an extended duration, depriving other network nodes of the bandwidth they may need. For example, a large virtual-machine system can include more than 100 gigabytes (GB) that must be migrated. Accordingly, there remains a need for a less burdensome approach to upgrading (or reverting, downgrading, cross-grading or otherwise updating or exchanging) hypervisors.